Bootstrap

<div class="dropdown">
    <button class="btn btn-secondary dropdown-toggle" type="button" id="dropdownMenuButton" data-toggle="dropdown"
        aria-haspopup="true" aria-expanded="false">
        Dropdown button
    </button>
    <div class="dropdown-menu" aria-labelledby="dropdownMenuButton">
        <a class="dropdown-item" href="#">Action</a>
        <a class="dropdown-item" href="#">Another action</a>
        <a class="dropdown-item" href="#">Something else here</a>
    </div>
</div>

<ul class="nav">
    <li class="nav-item">
        <a class="nav-link active" href="#">Active</a>
    </li>
    <li class="nav-item">
        <a class="nav-link" href="#">Link</a>
    </li>
    <li class="nav-item">
        <a class="nav-link" href="#">Link</a>
    </li>
    <li class="nav-item">
        <a class="nav-link disabled" href="#" tabindex="-1" aria-disabled="true">Disabled</a>
    </li>
</ul>

<button type="button" class="btn btn-primary">Primary</button>

<div class="alert alert-primary" role="alert">
    A simple primary alert—check it out!
</div>

<div class="alert alert-secondary" role="alert">
    A simple secondary alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

<div class="alert alert-success" role="alert">
    <h4 class="alert-heading">Well done!</h4>
    <p>Aww yeah, you successfully read this important alert message. This example text is going to run a bit longer so
        that you can see how spacing within an alert works with this kind of content.</p>
    <hr>
    <p class="mb-0">Whenever you need to, be sure to use margin utilities to keep things nice and tidy.</p>
</div>